I would like to thank all of you for valuable ideas. I will try to adapt some of them:
1) I see I was too cautious, which resulted in underdosing the necessary nutrients. As the first step, I will just try to increase the dosing of what I still have at home (EasyLife Nitro (which I stopped using in summer when I saw that I repeatidly measured 30 mg/l of NO3 in our tap water), EasyLife Fosfo, JBL Ferropol) and see what happens. I definitely improved CO2 dosing over last month and managed to drop pH to 7.3.
2) There is a chance that EDTA-chelated Ferropol will not work well due to the pH of 7.3. In that case I will order a new complete fertilizer (with DPTA chelator).
3) I will try to dose in abundance and perform the weekly "reset" by 50% WC. However, I must admit that I do not see it as a long term solution to exchange 150 liters of water on weekly basis....
4) The moment I see improvements, I will get some new plants to increase the plant mass.